The present invention relates generally to an apparatus and a method for controlling an elevator power supply receiving alternating current input power.
In order to make optimal use of the electrical energy available, present-day elevator drives are equipped almost exclusively with modern, that is digitally controlled power electronic components. With the wide spread use of static inverters, three-phase regulators and frequency inverters, the problems of the supply network influence by supply network reaction have increased greatly in the last decade. In particular the use of diode bridges, three-phase regulating units and three-phase rectifiers with phase gate control as input circuits are the actual supply line noise sources because they have strong nonlinear flows.
The problems caused by supply line reactions occur mainly in the form of higher harmonics and intermediate harmonics in the voltages and currents of the supply line and in the consumption of reactive power (kilovar).
In elevator systems, the non-sinusoidal currents are caused by power electronic components, such as for instance by static frequency inverters, the input circuits of which consist either of uncontrolled rectifier bridges or of thyristors. These higher harmonics in the currents produce harmonic voltages in the impedances in the supply network which heterodyne with the sinusoidal 50/60 Hz line voltage and distort the same--this results frequently in a flattening of the sine wave peaks.
The higher harmonics in the voltage can be noticed in all electrical components supplied by this network and cause an additional load on the equipment of the customer and the network supplier (power company). In case the higher harmonic voltages exceed the immunity to distortion of the connected equipment, disturbances are caused in the latter.
For this reason the owner of a reliable supply network must consider not only the interest of its customers but also, for the sake of undisturbed network operation, consider that the higher harmonic voltages and currents in the network should not exceed certain limit values.